HPC and Mainframe basically, but they both have quirks and extra bits required for the role due to the unique nature of the environments that never really exist

HPC and Mainframe basically, but they both have quirks and extra bits required for the role due to the unique nature of the environments that never really existed in traditional old school web based sysadmin work from the 90s and 2000s, and sometimes the title isn't Sysadmin. So basically you aren't walking into one of those roles cause you admin'd a bunch of Apache servers back in the day or anything like that. Point being, there is still a market for "pets above cattle" style infra work. It is just in increasingly specialized and technically challenging areas.